Coherent microwave-photon-mediated coupling between a semiconductor and a superconducting qubit

ARTICLE https://doi.org/10.1038/s41467-019-10798-6 OPEN Coherent microwave-photon-mediated coupling between a semiconductor and a superconducting qubit 1234567890():,; P. Scarlino1,5, D.J. van Woerkom1,5, U.C. Mendes2,4, J.V. Koski1, A.J. Landig1, C.K. Andersen C. Reichl1, W. Wegscheider1, K. Ensslin 1, T. Ihn1, A. Blais2,3 & A. Wallraff 1 1, S. Gasparinetti1, Semiconductor qubits rely on the control of charge and spin degrees of freedom of electrons or holes confined in quantum dots. They constitute a promising approach to quantum information processing, complementary to superconducting qubits. Here, we demonstrate coherent coupling between a superconducting transmon qubit and a semiconductor double quantum dot (DQD) charge qubit mediated by virtual microwave photon excitations in a tunable high-impedance SQUID array resonator acting as a quantum bus. The transmoncharge qubit coherent coupling rate (~21 MHz) exceeds the linewidth of both the transmon (~0.8 MHz) and the DQD charge qubit (~2.7 MHz). By tuning the qubits into resonance for a controlled amount of time, we observe coherent oscillations between the constituents of this hybrid quantum system. These results enable a new class of experiments exploring the use of two-qubit interactions mediated by microwave photons to create entangled states between semiconductor and superconducting qubits. 1 Department of Physics, ETH Zürich, CH-8093 Zürich, Switzerland. 2 Institut quantique and Department de Physique, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada. 3 Canadian Institute for Advanced Research, Toronto, ON, Canada. 4Present address: Instituto de Física, Universidade Federal de Goiás, Goiânia, Go CEP 74.690-900, Brazil. 5These authors contributed equally: P. Scarlino, D.J. van Woerkom. Correspondence and requests for materials should be addressed to P.S. (email: ) NATURE COMMUNICATIONS | (2019)10:3011 | https://doi.org/10.1038/s41467-019-10798-6 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-10798-6 S ingle electron spins confined in semiconductor quantum dots (QDs) can preserve their coherence for hundreds of microseconds in 28Si1,2, and have typical relaxation times of seconds3,4. This property can be explored, for example, to build memories for quantum information processors in hybrid architectures combining superconducting qubits and spin qubits. Typically, semiconductor qubit–qubit coupling is short range, effectively limiting the interqubit distance to the spatial extent of the wavefunction of the confined particle, which is a significant constraint toward scaling to reach dense 1D or 2D arrays of QD qubits. Strategies to interconnect semiconductor qubits include the control of short-range interactions through the direct overlap of electronic wavefunctions5–7, the direct capacitive coupling between QDs8, enhanced by floating metallic gates9, shuttling of electrons between distant QDs by surface acoustic waves10,11, by time-varying gate voltages12 and by fermionic cavities13. An alternative approach which allows for long-range qubit–qubit interaction, inspired by superconducting circuit quantum electrodynamics (QED)14, and recently explored also for semiconductor QDs15–17, is to use microwave photons confined in superconducting resonators to mediate coupling between distant qubits. In this approach, the microwave resonator not only acts as a quantum bus, but also allows for quantum nondemolition qubit readout18–20. With the well established strong coupling of superconducting qubits to microwave resonators14 and the recently achieved strong coupling to charge states in semiconductor double dot structures21,22, it is now possible to create a microwave photonbased interface between superconducting and semiconducting qubits mediated by a joint coupling resonator. A similar strategy has been explored in hybrid structures interfacing a transmon qubit with excitations of a spin-ensemble of NV centers in diamonds23–25 and of collective spins (magnons) in ferromagnets26–28. Furthermore, direct coupling between a superconducting flux qubit and an electron spin ensemble in diamond was investigated29. In these works the strong coupling regime was achieved with ensembles, for which the coupling strength scales with the square root of the number of two-level systems interacting with the resonator mode. Here, we explore the coupling of the charge degree of freedom of a single electron confined in a double QD (DQD) to a superconducting transmon qubit in the circuit QED architecture14. The coherent coupling between dissimilar qubits over a distance of a few hundred micrometers is mediated by virtual microwave photon excitations in a high impedance SQUID array resonator, which acts as a quantum bus. We demonstrate resonant and dispersive interaction between the two qubits mediated by real and virtual photons, respectively. We extract a coupling strength of ~36 MHz (~128 MHz) between the bus resonator and the DQD (transmon) around the frequency of ~3.7 GHz. With a frequency detuning of ~370 MHz from the resonant frequency of the bus resonator, we spectroscopically observe a qubit avoided crossing of about ~21 MHz. The strength of the virtual-photon mediated interaction is extracted from measurements of coherent qubit population oscillations. The methods and techniques presented here have the potential to be transferred to QD devices based on a range of material systems and can be beneficial for spin-based hybrid systems. Results Sample design and basic circuit characterization. To perform our experiments, we integrate four different quantum systems into a single device: a semiconductor DQD charge qubit, a superconducting qubit, and two superconducting resonators (see Fig. 1a). One resonator acts as a quantum bus between the 2 superconducting and the semiconductor qubits and the other one as a readout resonator for the superconducting qubit. In this way, the functionality for qubit readout and coupling is implemented using two independent resonators at different frequencies, allowing for more flexibility in the choice of coupling parameters and reducing unwanted dephasing due to residual resonator photon population30. A simplified circuit diagram of the device is shown in Fig. 1f. The superconducting qubit is of transmon type and consists of a single superconducting aluminum (Al) island shunted to ground via a SQUID (orange in Fig. 1). The transmon charging and Josephson energies are Ec/h ~243.0 ± 0.2 MHz and EJ0 =h
30:1 ± 0:1 GHz, respectively (see Supplementary Note 2 for more information). The transition frequency ωtr between its ground state |g〉 and excited state |e〉 is adjusted by using the magnetic flux generated in the transmon SQUID loop by a flux line (purple in Fig. 1). We read out the state of the transmon qubit with a 50 Ω coplanar waveguide resonator (dark blue in Fig. 1) capacitively coupled to the qubit14,31. The DQD charge qubit (Fig. (...truncated)